Gastrointestinal infections are killing more and more people in the United States and have become a particular threat to the elderly, according to data released last week.

Deaths from the infections more than doubled from 1999 to 2007 -- from 7,000 a year to more than 17,000 a year, the Centers for Disease Control and Prevention reported. Of those who died, 83 percent were over age 65.

Two-thirds of the deaths were caused by a bacterium, Clostridium difficile, that people often contract in hospitals and nursing homes, particularly when they have been taking antibiotics. The bacteria have grown increasingly virulent and resistant to treatment in recent years.

But researchers were surprised to discover that the second leading cause of death from this type of illness was the norovirus. It causes a highly contagious infection, sometimes called "winter vomiting illness," that can spread rapidly on cruise ships and in prisons, dormitories and hospitals.

"I think there is perhaps a misperception that norovirus causes a mild illness," said Aron Hall, a CDC epidemiologist. "But this suggests a major problem that requires some attention."

Both diseases are spread by the fecal-oral route, meaning that people swallow germs found in feces, often spread by people who did not wash their hands after using the toilet.